Manufacturers and wholesalers use flexible thermoplastic packaging films to provide economical, sanitary containers, which help protect and/or preserve the freshness and wholesomeness of their products. These films are often sold in bag (or pouch) form. For example, a single or multilayer film is made into bags or pouches using a tubular film or one or more flat sheets or webs of film by well known processes involving e.g. cutting, folding and/or sealing the film to form bags and pouches. The term pouch is often used to refer to a bag made from at least two separate sheets of film or having two heat seals leaving a single open side whereas bag is often used to refer to tubular films that have a single seal transverse to the machine direction of the tube leaving an open side or generically to also include pouches or any packaging having a, typically single, open side. These films and bags may be printed and may also be uniaxially or biaxially oriented, heat shrinkable, irradiated, or may contain film layers which are abuse resistant or puncture resistant or which are crosslinked or which enhance or retard or prevent transmission of light, gases, or liquids therethrough. Frequently, multilayer films having one or more barrier layers to oxygen and/or moisture such as: saran (a polyvinylidene chloride copolymer); a modified saran e.g. MA-saran containing methyl acrylate polymer units; ethylene vinyl alcohol (EVOH) copolymer; nylon; or acrylonitrile may be used with a heat sealing layer such as a copolymer of ethylene and vinyl acetate (EVA), ethylene alpha-olefin copolymer or blends thereof to produce bags for packaging oxygen and/or moisture sensitive foods e.g. processed pork or fresh red meat. Such bags help preserve meat in its original condition by preventing or reducing moisture loss and chemical changes in the meat structure due to oxidation reactions.
A typical package has one, two or three sides heat-sealed by the bag manufacturer leaving one open side to allow product insertion. For example, a processor may insert fresh, frozen or processed meat, ham, poultry, cheese, primal or subprimal meat cuts, ground beef, fruits, vegetables, bread or other products making a final seal to hermetically enclose the product in the bag. This final seal may follow gas evacuation (i.e. vacuum removal) or replacement of the gaseous environment within the bag by one or more gases to provide some advantage such as to assist product preservation. This final seal is frequently a heat seal similar to the initial seals produced by the bag manufacturer although the actual heat sealing equipment may vary.
Thus, bags are made: by transversely sealing tubular stock of monolayer or multilayer film and cutting off the tube portion containing the sealed end; by making two spaced apart transverse seals on tubular stock and cutting open the side of the tube; by superimposing flat sheets of film and sealing on three sides; or by folding a flat sheet and sealing two sides.
After a product is inserted, the bag is typically evacuated and the bag mouth sealed to enclose the product. At one time, the standard method for sealing was to fasten a clip around the mouth of the bag. However, heat sealing techniques are now also commonly employed to produce the final closure of the bag. For example, a bag mouth may be either hot bar sealed or impulse sealed. An impulse seal is made by application of heat and pressure using opposing bars similar to the hot bar seal except that at least one of these bars has a covered wire or ribbon through which electric current is passed for a very brief time period (hence the name “impulse”) to cause the adjacent film layers to fusion bond. Following the impulse of heat the bars are typically cooled (e.g. by circulating coolant) while continuing to hold the bag inner surfaces together to achieve adequate sealing strength. Other sealing techniques are well known in the art and are described e.g. in The Wiley Encyclopedia of Packaging Technology, 2nd Ed., Brody & Marsh editors, pp. 823-827 (John Wiley & Sons, Inc., 1997) which is hereby incorporated by reference.
There must be a strong continuous seal to prevent unwanted egress and ingress of gaseous, liquid, or solid materials between the bag exterior and interior. This is particularly necessary when the package is made of heat shrinkable film and is to be heat shrunk e.g. by being immersed in hot water to shrink the film against the packaged article since such shrinkage increases the stress on these seals. It is even more critical where the packages are to be thermally processed at elevated temperatures i.e. heated at sufficient times and temperatures for pasteurization or cooking or surface pasteurization (sometimes referred to as surface post-pasteurization). Thus, there is a continuing need for films which can be made into bags having strong seals especially those formed by hot bar and/or impulse sealing. Such films should provide strong seals able to withstand a range of temperatures and also be able to make such seals over a wide sealing temperature range without burn through of the seal area which can lead to weak seals and/or seal failure.
A very demanding application for heat shrinkable, heat sealable thermoplastic flexible films is for thermally processing meats. Bacterial contamination during food processing e.g. by Listeria monocytogenes is of great concern. To address health and safety concerns with processed foods, some processors have adopted a surface heat treatment at elevated temperatures sufficient to kill bacteria on already cooked food.
In some demanding applications, a food product such as a ham is sealed inside a plastic processing bag or film in which the ham is cooked, refrigerated, shipped, and subsequently displayed for retail sale.
In a more common demanding application, food such as a turkey breast, ham, or beef is cooked in a pan, net, or processing film from which the cooked food is removed for further processing such as: slicing; smoking in a smokehouse; treatment with colorants and/or flavorants such as caramel, spices, liquid smoke, or honey; glazing; and/or removal of liquid (known as purge) resulting from e.g. the cooking process. Following this further processing, the food product is packaged, often in a printed bag, for shipment and sale. The cooked food is typically placed into a heat sealable, heat shrinkable bag which is then emptied of atmospheric gases by vacuum, heat sealed and subjected to a film shrinking operation usually in a water tank at elevated temperature for a brief period of time to produce a compact attractive package. During these steps that follow cooking and occur prior to packaging for shipment, and sale, the food product surface is subject to environmental contamination, for example, by airborne particles, microbes, and dust. The risk from contamination after packaging may be minimized by surface pasteurizing the encased sealed package, e.g., in a water bath or steam chamber held at elevated temperatures (and optionally at elevated pressures) for a time sufficient to provide the desired degree of protection from microbial contamination and growth. The time, temperature, and pressure of this post-cooking pasteurization step may vary widely.
Significantly, this surface treatment is in addition to the cooking or pasteurization process and follows hermetically sealing the cooked or pasteurized food in a plastic packaging film. In this demanding use, this “post-cooking pasteurization” surface treatment is performed after placing the food into the packaging film that will remain on the pasteurized product through sale to an ultimate customer. Often the films are printed with consumer information and brand identification and frequently at least a portion of the film is clear to allow viewing of the enclosed product. Therefore, optical properties and film appearance are important for consumer appeal and sale.
This “post-pasteurization” film must perform a variety of functions well. It must be puncture resistant and have strong seals at the elevated temperatures encountered in the shrinking operation, and also with the post-cooking pasteurization process. It should also keep tight conformation of the film around the product at refrigeration temperatures with an attractive appearance and act as a good barrier to oxygen, moisture, and environmental contaminates.
One particular problem during heat sealing the film of a package for cook-in or other thermal processing applications is that of excessively high tear propagation strengths. Although the strong heat seals provide protection against unwanted seal failure, such seals also make it difficult for the end user to open the package. Accordingly, there is a need for an improved film for a heat-shrinkable package that includes seals of sufficient peel and shear strength to survive the heat shrinking and cook-in or post-cooking pasteurization shrink forces, yet be readily openable by the end user without the use of a knife or cutting implement, and without uncontrolled or random tearing or rupturing of the packaging materials.